Github user nsyca commented on the issue:
https://github.com/apache/spark/pull/17491
@cloud-fan wrote: "How useful is this optimization? It only works when
Exists has no condition, is that a common case?"
One of the common cases of this usage is an application of ACL where the
application asks the database whether the user has a proper authority to access
a certain set of data or not.
Ex:
select ... from controlled_table where exists (select 1 from acl_table
where user = CURRENT_USER and role = ...)
From a runtime perspective, an optimal access plan is placing the ACL_TABLE
as an outer of a nested-loop join with a semantic to fetch only the first
qualified row, once the row exists, continue to process the inner table,
CONTROLLED_TABLE, or avoiding access the inner completely if no qualified row
from the outer.
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